6.37. In addition, in the development of some waste disposal facilities, hypothetical ‘what if’ scenarios have been used (see para. 5.39). These assume, for example, the absence of a particular barrier or safety function or, for near surface disposal facilities, the loss of institutional control during the assumed period of institutional control. Such scenarios allow investigation of the response of the disposal system to perturbations, even if such perturbations are considered hypothetical. However, despite the assumptions made in the ‘what if’ scenario concerning the absence of a certain safety function, efforts still need to be expended to ensure this safety function will continue to be fulfilled, as it is an element of the defence in depth for the facility.